Fertilizers have been used for many years to supplement nutrients in growing media. In recent years the art has focused on techniques to deliver controlled amounts of plant nutrients to the soil or other growing media. It is recognized, for example, that controlling the release of plant nutrients such as nitrogen from highly soluble fertilizer granules is desirable because releasing the nutrients over an extended period of time achieves advantages which include increased efficiency of fertilizer use by plants, reduced application costs since fewer applications of fertilizer are required and reduced nutrient loss caused by leaching and denitrification. The coating of sulfur on the surface of the fertilizer granules may reduce the dissolution rate of the granules and impart controlled-release characteristics. In essence, the water in the soil, and rain water are kept away from the very soluble fertilizer until a granule develops a flaw such as a crack or fissure in the coating, or the coating is penetrated by microbial action.
Due to the inherent brittleness of the crystalline solid sulfur-coating which forms on the granule, and the thin, or even non-continuous coating on many of the granules, it is essential that some type of secondary outer coating or sealant be applied onto the sulfur-coated surface. This sealant conventionally is either a polymeric hydrocarbon, petroleum-based wax, or a combination of high viscosity polymeric paraffinic oil plus polyethylene, which is spray applied as a hot melt liquid onto the hot, but solidified sulfur-coating surface. Although these sulfur coated fertilizers have received substantial uses, there are problems from the standpoint of obtaining uniform coating thicknesses, predictable release characteristics resulting from cracks in the sulfur coatings, essential abrasion and impact resistance, and the complexity of the processing steps.
More recently, because of problems associated with sulfur coated fertilizers, such as above defined, polymer coated fertilizers have received substantial attention, particularly in view of the improved controlled release properties obtained with certain polymer coated fertilizers at lower coat weights.
U.S. Pat. No. 5,538,531 (Hudson) teaches a controlled release, particulate fertilizer product having a water soluble fertilizer central mass encased in a plurality of water insoluble, abrasion resistant coatings. At least one inner coating is a urethane reaction product derived from reacting recited isocyanates and polyols. The outer coating is formed from an organic wax.
U.S. Pat No. 6,358,296 (Markusch et al.) teaches a slow-release polyurethane encapsulated fertilizer using oleo polyol(s). U.S. Pat. No. 5,851,261 (Markusch et al.) provides a process for the production of polyurea encapsulated fertilizer particles comprising applying an isocyanate-reactive component containing at least two amine groups to the fertilizer particles, and applying a polyisocyanate to the amine coated particles to form polyurea coated particles.
Sulfur containing isocyanate compositions and a process for the production of encapsulated fertilizer compositions are described in U.S. Pat. No. 6,152,981 (Markusch et al.). The fertilizer compositions are prepared by applying a mixture of sulfur and an isocyanate to the fertilizer and then applying an isocyanate-reactive material. U.S. Pat. No. 5,599,374 (Detrick) describes a fertilizer composition wherein a sulfur coating is applied to a fertilizer core, and thereafter a polymer coating is applied over the sulfur.
U.S. Pat No. 6,231,633 (Hirano et al.) teaches a granular fertilizer coated with a thermosetting resin coating that may be urethane and a hydrophobic compound, which may be wax. U.S. Pat. No. 6,663,686 (Geiger et al.) teaches a slow-release polyurethane encapsulated fertilizer using polyurethane and wax.
It is also known in the art to pre-coat particulate plant nutrient with organic oil and particles as a means to improve the release profiles of the particulate plant nutrient (U.S. Pat. No. 6,039,781; Goertz et al.).
U.S. Pat. No. 6,338,746 (Detrick et al.) describes a process of first coating a fertilizer with a polymer, then coating the polymer with sulfur and thereafter applying a polymer coating. The polymers are described in U.S. Pat. Nos. 4,711,659 (Moore), 4,804,403 (Moore) and 5,374,292 (Detrick). These polymers require that the substrate contains a minimum quantity of reactive —NH2 groups. Thus, these are not applicable to all fertilizer compositions for which slow release properties may be desirable.
Although polymer coated fertilizers as above described have received substantial attention, they are expensive to manufacture. There is a need in the art to provide controlled released fertilizer formulations that are abrasion resistant, and that reduce the cost of fertilizer production.